Method for decomposing N20 utilizing catalysts comprising calcined anionic clay minerals

Abstract: The invention describes a highly efficient catalytic pollution control process for removing N₂O from gaseous mixtures. The process utilizes catalysts derived from anionic clay minerals, which after appropriate heat activation, provide superior N₂O decomposition activity. The catalytic process comprises contacting an N₂O-containing gaseous mixture with the decomposition catalyst under conditions sufficient to convert the N₂O into gaseous nitrogen and gaseous oxygen. The process catalysts are derived from anionic clay materials such as the hydrotalcites, sjogrenites and pyroaurites.

Description: TECHNICAL FIELD OF THE INVENTION The present invention describes a highly efficient pollution control process for catalytically converting nitrous oxide (N 2 O) into nitrogen gas and oxygen gas which utilizes decomposition catalysts derived from anionic clay minerals, which after appropriate heat activation, provide superior N 2 O decomposition activity. BACKGROUND OF THE INVENTION Nitrous oxide (N 2 O) is not commonly considered an atmospheric pollutant and has not been considered a constituent of the gaseous pollutants collectively referred to as nitrogen oxides (NO x ) which have received wide attention as pollutants harmful to the environment. However, recent studies indicate that N 2 O in the Earth's atmosphere may be increasing at a rate of about 0.2% per year and that this increase appears to be caused by anthropogenic activity. N 2 O is a major stratospheric source of NO, is believed to be involved in destroying the ozone layer and is recognized to be a green-house gas. Because N 2 O has an atmospheric lifetime of approximately 150 years, researchers are attempting to identify sources of the pollutant and to limit further production of the harmful gas. Recent reports such as an article by Thiemens and Trogler, Science , 251 (1991)932 suggest that various industrial processes significantly contribute to the increased levels of N 2 O found in the Earth's atmosphere. For example, nitrous oxide is a by-product formed during the manufacture of monomers used in producing 6,6- and 6,12-nylon. Approximately 1.24 X 10 9 kg of nylon were produced in the United States in 1988, alone. Nylon polymers are typically formed by subjecting a dicarboxylic acid and a diamine to a condensation polymerization reaction. The most widely used dicarboxylic acid, adipic acid, is prepared primarily by oxidizing cyclohexane in air to form a cyclohexanol/cyclohexanone mixture followed by oxidizing such mixture with HNO 3 to form adipic acid and N 2 O. Thiemens and Trogler calculate that about 1 mol of N 2 O per mole of adipic acid is formed as a side product in adipic acid processes. Assuming that 2.2 x 10 9 kg of adipic acid are produced globally per year, about 1.5 x 10 10 mol yr -1 of N 2 O by-product or 10% of the annual output of atmospheric N 2 O can be attributed to this single process. M. Schiavello and coworkers, ( J. Chem. Soc. Faraday Trans. 1, 71(8), 1642-8 ) studied various magnesium oxide-iron oxide and magnesium oxide-iron oxide-lithium oxide systems as N 2 O decomposition catalysts. While magnesium oxide-iron oxide samples which were fired in air and which contained MgFe 2 O 4 demonstrated low activity, similar samples fired under reducing atmospheres and containing Fe 2+ in solid solution demonstrated greater activity. The researchers concluded that Fe 3+ ions in the ferrite phase are not catalytically active toward the subject reaction whereas Fe 3+ ions contained in MgO together with Li + are catalytically active when the ratio of lithium to iron is less than 1. P. Porta and coworkers (J. Chem. Soc. Faraday Trans. 1, 74(7), 1595-603) studied the structure and catalytic activity of Co x Mg 1-x Al 2 O 4 spinel solid solutions for use as catalysts in decomposing N 2 O into gaseous nitrogen and oxygen. The catalytic activity per cobalt ion in various N 2 O decomposition catalysts was found to increase with increasing dilution in MgO. The distribution of cobalt ions among octahedral and tetrahedral sites in the spinel structure of Co x Mg 1-x Al 2 O 4 was found to vary with temperature and the fraction of cobalt ions in octahedral sites was found to increase with increasing quenching temperature. The researchers concluded that catalyst activity generally increases as a greater amount of cobalt ions is incorporated into octahedral sites in the structure. W. Reichle ( Journal of Catalysis 94 (1985) 547) reported that various anionic clay minerals belonging to the pyroaurite-sjogrenite group, such as hydrotalcite (Mg 6 Al 2 (OH) 16 (CO 3 2- )·4H 2 O can be thermally decomposed to form a product which is a useful catalyst for vapor-phase aldol condensations. Replacement of Mg by Fe, Co, Ni and Zn an/or replacement of Al by Fe and Cr also results in isomorphous double hydroxides which, on heat treatment, are rendered catalytically active. The reference also states that the activity of the catalyst is strongly affected by the temperature at which the hydrotalcite is activated. US-A-5 039 645 discloses the use of hydrotalcite as a support material in a catalyst for oxidizing N 2 O to NO 2 . U.S. Patent 5,171,553, assigned to Air Products and Chemicals, Inc., Allentown, Pennsylvania, discloses a highly efficient, commercially viable process for removing N 2 O from gaseous mixtures. The process utilizes catalysts comprising a crystalline zeolite which, at least in part, is composed of five membered rings having a structure type selected from the group consisting of BETA, MOR, MFI, MEL and FER wherein the crystalline zeolite has been at least partially ion-exchanged with a metal selected from the group consisting of copper, cobalt, rhodium, iridium, ruthenium and palladium. Industry urgently desires to develop catalytic processes for destroying N 2 O emissions prior to venting commercial process effluent streams into the atmosphere. Although catalytic decomposition of N 2 O has been studied extensively in academic institutions, few commercially viable processes are known for decomposing N 2 O into its respective components, namely gaseous nitrogen and gaseous oxygen, which utilizes a catalyst which exhibits activity and life provided by the catalysts of the present invention. BRIEF SUMMARY OF THE INVENTION The present invention is a catalytic process for removing N 2 O from exhaust streams which utilizes a catalyst formed from one or more anionic clay minerals, which after appropriate heat activation, provide superior N 2 O decomposition activity. The process comprises reacting an N 2 O-containing stream in the presence of an effective amount of one or more of the enumerated catalysts under conditions sufficient to convert the N 2 O to gaseous nitrogen and gaseous oxygen. The catalysts according to the present invention are formed by heat treating one or more anionic clay materials represented by the generic formula: M m N n (OH) (2m + 2n) A a ·bH 2 O wherein M is a divalent metal cation; N is a trivalent metal cation; A is a mono-, di- or trivalent anion which decomposes when heated to a temperature sufficient to form a volatile gas; m and n are such that m/n has values of 0.5 to about 6; a is a number with the provisos that when A is a divalent anion, a = 1/2n and when A is a trivalent anion, a = 1/3n and b is an integer having values of 1 to 10. Applicants' process utilizes a family of catalysts which has not been utilized for the subject N 2 O decomposition process. Furthermore, the invention utilizes catalysts which provide superior conversion of N 2 O to gaseous oxygen and nitrogen and extended catalyst life. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates conversion of N 2 O to gaseous nitrogen and oxygen obtained as a function of time and temperature using a claimed N 2 O decomposition catalyst. FIG. 2 illustrates conversion of N 2 O to gaseous nitrogen and oxygen obtained over extend time periods using a claimed N 2 O decomposition catalyst. FIG. 3 illustrates conversion of N 2 O to gaseous nitrogen and oxygen obtained as a function of time using a claimed N 2 O decomposition catalyst versus specified prior art cobalt-containing zeolite catalysts. FIG. 4 illustrates the surface area of various calcined anionic clay materials obtained as a function of catalyst calcination temperature. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a highly efficient catalytic process for converting nitrous oxide (N 2 O) into environmentally safe products, namely gaseous oxygen and gaseous nitrogen. The claimed process utilizes catalysts derived from one or more anionic clay minerals, which after appropriate heat activation, provide superior catalytic activity and life under typical process conditions. Moreover, the process can be operated at lower temperatures than processes using other catalysts and the catalysts used in the process exhibit extended catalyst life. The catalysts according to the present invention are formed by heat treating or calcining one or more anionic clay materials represented by the formula: M m N n (OH) (2m + 2n) A a ·bH 2 O wherein M is a divalent metal cation; N is a trivalent metal cation; A is a mono-, di- or trivalent anion which decomposes when heated to a temperature sufficient to form a volatile gas; m and n are such that m/n has values of 0.5 to about 6; a is a number with the provisos that when A is a divalent anion, a = 1/2n and when A is a trivalent anion a = 1/3n and b is an integer having values of 1 to 10. The resulting heat treated anionic clay materials possess a composition and morphology which is highly active toward decomposing N 2 O into its corresponding decomposition products, gaseous oxygen and nitrogen. The anionic clay materials recited in the above-mentioned formula include, but are not limited to materials commonly referred to as hydrotalcites, sjogrenites and pyroaurites which compositions are well defined and can be prepared according to methods known in the art. The temperature required to decompose the mono-, di- or trivalent anion, A, to form a volatile gas will vary depending upon the particular anionic clay material and such temperatures can be readily determined by one of ordinary skill in the art without undue experimentation. Typical decomposition temperatures range from about 200° to 800°C. The decomposition temperature is not critical to the practicing the invention. The anionic clay minerals from which the process catalysts are derived can be prepared synthetically utilizing a variety of anions. Moreover, the M +2 /N +3 ratio can be varied widely to achieve the desired catalyst activity. The anionic clays minerals of the present invention are converted to an active catalytic material by heating the anionic clay material to a temperature sufficient to form a catalyst which exhibits N 2 O decomposition activity. Suitable heat treatment temperatures vary depending upon the particular anionic clay material and usually range from about 200°C to about 800°C at which temperatures the anionic moieties are decomposed to gaseous compounds. The heat treatment can be conducted in air, inert gases, vacuum or in an N 2 O-containing gaseous mixture. The preferred ratio of m to n according to the formula of the general embodiment can be varied from 1 to 4 according to conventional techniques known in the art. M can be any divalent cation which provides N 2 O decomposition activity upon heat treating the anionic clay material. Suitable divalent cations include Co 2+ , Cu 2+ , Ni 2+ , Pd 2+ , Zn 2+ , Fe 2+ , Mn 2+ , Mg 2+ and mixtures thereof. Suitable trivalent cations include Al 3+ , Ga 3+ , In 3+ , Fe 3+ , Rh 3+ , Co 3+ , B 3+ , Cr 3+ , La 3+ , Sc 3+ , Y 3+ and the rare earth metals including Ce 3+ , Nd 3+ , Pm 3+ , Sm 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ , Er 3+ , Tm 3+ , Yb 3+ , Lu 3+ and mixtures thereof. According to a preferred embodiment, the process utilizes catalysts formed by heat treating one or more anionic clay materials represented by the formula: M m Al 3+ n (OH) (2m + 2n) A a ·bH 2 O wherein M is Co 2+ , Cu 2+ , Ni 2+ , Pd 2+ , Mg 2+ or mixtures thereof; A is a mono-, di- or tri-valent anion which decomposes when heated to a temperature sufficient to form a volatile gas; m and n are such that m/n has values of 0.5 to about 6; a is a number with the provisos that when A is a divalent anion, a = 1/2n and when A is a trivalent anion a = 1/3n and b is an integer having values of 1 to 10. Throughout the Specification and Claims, Applicants shall use the abbreviation, HT, for hydrotalcite. Since each trivalent cation has one more positive charge than the divalent cation, the sheets gain one unit of positive charge per each trivalent ion which charge is compensated by suitable anions such as CO 3 2- which are located interstitially. Water molecules may be situated between each metal ion sheet. The preferred ratio of M to Al according to the formula of the preferred embodiment can be varied from 1.5 to 4 according to conventional techniques known in the art. While Mg-Al-HT is essentially inactive toward catalytic decomposition of N 2 O, superior catalytic activity is obtained by utilizing the claimed catalytic compositions wherein the enumerated divalent and trivalent cations are incorporated into the anionic clay materials. Suitable divalent cations include Co 2+ , Cu 2+ , Ni 2+ , Pd 2+ , Zn 2+ , Fe 2+ , Mn 2+ , Mg 2+ and mixtures thereof. Suitable trivalent cations include Al 3+ , Ga 3+ , In 3+ , Fe 3+ , Rh 3+ , Co 3+ , B 3+ , Cr 3+ , La 3+ , Sc 3+ , Y 3+ and the rare earth metals including Ce 3+ , Nd 3+ , Pm 3+ , Sm 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ , Er 3+ , Tm 3+ , Yb 3+ , Lu 3+ and mixtures thereof. Preferred divalent cations include Co 2+ , Ni 2+ , Cu 2+ , Pd 2+ , Zn 2+ and Fe 2+ and preferred trivalent cations include Al 3+ , Ga 3+ , Fe 3+ , Rh 3+ , Co 3+ and La 3+ . Such divalent or trivalent cations can be incorporated into the anionic clay materials by the methods described herein to form particularly active catalysts. As shall be explained in greater detail, a portion of the divalent ion enumerated in the formulae of the general and preferred embodiments, such as Mg 2+ in Mg-Al-HT, can be completely or partially replaced by other divalent cations such as copper, cobalt, zinc, iron, nickel or palladium. Optionally, a portion of the aluminum atoms can be replaced by Cr, V, La, Rh and Fe to yield active catalysts for N 2 O decomposition. The active catalysts of the present invention are derived from layered mixed metal hydroxides having a divalent ion which is partially substituted by trivalent ions whose excess positive charge is compensated by anions occupying the interlayer positions. Techniques capable of preparing the anionic clay materials of the general and preferred embodiments are available in the public domain. For example, suitable methods include the synthetic techniques described by S. Miyata and coworkers, Nippon Kagaku Zasshi 92 (1971) 514; ion exchange techniques described by S. Miyata and coworkers, Clays Clay Miner . 31 (1983) 305; and techniques involving neutralization of the respective carbonates described by D. L. Bish, Bull. Mineral 103 (1980) 170. For example, Reichle ( Solid States Ionics 22 (1986) 135) teaches several methods for preparing anionic clay minerals suitable for practicing the enumerated process. A general method for preparing such materials involves mixing a concentrated aqueous solution of the desired cationic metal sources (represented by M 2+ and M 3+ in the formula of the general and preferred embodiments) with an aqueous hydroxide/carbonate solution to yield an amorphous gel which is thermally aged at a temperature between 60° to 325°C to provide an active catalyst for processes other than N 2 O decomposition. Alternately, the anionic clay materials of the present invention can be prepared by conventional methods proposed by Reichle and coworkers ( J. Catal ., 94 (1985) 547) wherein desired metal nitrates are sequentially precipitated using a NaOH/Na 2 CO 3 mixture. The resultant precipitate is aged at 65°C for 24 hours and is filtered, washed with distilled water and dried overnight at 70°C to provide the desired anionic clay powder. An alternate method for preparing the enumerated anionic clay materials, in general, and HT, in particular, comprises adding a solution of mixed magnesium/aluminum nitrates, sulphates or chlorides to a stoichiometric solution of sodium hydroxide and sodium carbonate at about 25° to 35°C, while stirring vigorously over a period of several hours to form a slurry. The slurry is heated to about 50° to 200°C for 1 to 18 hours (preferably 60° - 75°C) in order to promote crystallization. A practical mixing temperature ranges from about 65° - 75°C which yields a crystal of about 150 -300 Å in size having a surface area of 100-120 m 2 /g (BET/N 2 technique). The resulting solids are recovered by filtering followed by washing and drying. The rate is which the metal ions are added to the aqueous caustic/carbonate solution is not critical to preparing the materials nor is the reaction temperature. The rate of addition can also be varied widely. However, the solutions must be effectively agitated in order to preclude undesired reactions from taking place. The addition temperature is normally maintained at or about room temperature. The resulting gel material is aged at elevated temperatures, normally less than 100°C, to form hydrotalcite crystallite. Applicants have discovered that the anionic clay materials should not be washed excessively prior to heat treatment because such excessive washing results in diminished catalyst activity. Applicants believe that excessive washing of the anionic clay precursor removes alkali metal or alkaline earth metal from the material. Applicants have further discovered that materials having diminished alkali metal or alkaline earth metal content due to excessive washing can be modified by adding small amounts of the depleted alkali metal salts or alkaline earth metal salts to the material prior to calcination to provide a catalyst which exhibits enhanced activity toward N 2 O decomposition. For example, sodium salts, potassium salts, lithium salts and calcium salts can be used for this purpose. The resulting materials represented by the formulae of the general and preferred embodiments are heat treated at a temperature between about 150° and 800°, preferably between 300° and 500°C, in air, an inert gas stream, in vacuo or in situ in the presence of the N 2 O-containing stream to be decomposed to provide a catalyst which possesses a well defined X-ray powder pattern. The heat treatment temperature used to convert the anionic clay material to the active catalyst can be varied widely. At temperatures less than about 200°C, the desired mixed metal oxides are formed slowly and incompletely. The heat treatment temperature should be selected to provide to control the extent to which the anionic clay material is converted to active catalyst while maintaining acceptable surface area. Preferred temperatures for heat treating a given anionic clay material according to the present invention can be readily determined without undue experimentation. Non-stoichiometric, well dispersed mixed metal oxides are formed upon heat treating the enumerated anionic clay materials within the specified temperature ranges. The heat treatment causes the mono-, di- and trivalent anions represented by "A" in the formula of the general and preferred embodiments to decompose to a volatile gas thereby forming the active catalyst. The fine catalyst powders obtained according to these methods can be supported on conventional inorganic supports such as clays, alumina, silica, silica/alumina, titania, zeolites or cordierite. The powders can be formed into pellets or extruded to form particles which are wear and impact resistant and can function effectively in fixed-bed reactors. Conventional binders and other additives can be added to the catalyst to aid in the forming process. The process for destroying nitrous oxide comprises contacting a gaseous mixture containing nitrous oxide with a desired catalyst according to the present invention under conditions sufficient to convert the nitrous oxide to gaseous nitrogen and gaseous oxygen. The amount of catalyst to be utilized in the present process varies depending upon the reaction conditions (i.e., temperature, pressure and the like), and the amount of N 2 O residing in the process stream to be treated. Desirably, an effective amount of catalyst is used, i.e., that amount which causes a reaction to occur when N 2 O is placed in contact with the enumerated catalysts under the specified reaction conditions. The catalysts of the invention can be fabricated onto high temperature supports known in the art including those customarily used in mobile and stationary emission control applications. The process of this invention can be utilized to remove N 2 O from essentially any gaseous stream containing N 2 O and the process is operative even when appreciable amounts of hydrocarbons, NOx, water and oxygen are present in the N 2 O-containing gaseous mixture. The process according to this invention is typically run at temperatures ranging from about 100°C to 800°C and pressures between about 0.1 and 100 atmospheres. However, higher temperatures may be utilized because the maximum temperature for conducting the process is limited only by the temperature stability of the catalyst itself. More particularly, the process can be advantageously run under fixed bed conditions at temperatures ranging from about 100°C to 800°C and a gas hourly space velocity ranging from 1,000 to 300,000 hr -1 , preferably 7,500 hr -1 to 30,000 hr -1 . FIG. 1 illustrates conversion of N 2 O to gaseous nitrogen and oxygen obtained as a function of time and temperature using a cobalt-aluminum hydrotalcite catalyst which was calcined at 500°C according to the present invention. FIG. 1 indicates that N 2 O decomposition activity of the subject catalysts is not markedly affected by exposure to a simulated water-containing N 2 O process stream (15% N 2 O, 2% H 2 O and the balance helium) over a 30 hour period. 0.25g of the catalyst was utilized at a space velocity of 18,000 hr -1 . FIG. 1 also illustrates that conversion of N 2 O to gaseous nitrogen and oxygen approaches 100% at a process temperature of 650°C and approaches 65% at a process temperature of 475°C and that such a desired conversion level can be achieved by adjusting the process operating temperature or space velocity. FIG. 2 illustrates conversion of N 2 O to gaseous nitrogen and oxygen obtained as a function of time using a cobalt-aluminum hydrotalcite catalyst which was calcined at 500°C. A larger bed of catalyst was used in this experiment compared to the experiment according to FIG. 1. While the process temperature was initially maintained at 500°C, the temperature of the catalyst bed increased to about 670°C due to the exothermic nature of the reaction. FIG. 2 indicates that activity of the subject catalyst was not markedly affected by oxygen or moisture in N 2 O containing gaseous mixture over the 175 hour test period. The Process stream contained 10% N 2 O, 2% H 2 O, 2% O 2 and the balance helium. FIG. 2 illustrates that conversion of N 2 O to gaseous nitrogen and oxygen approaches 100% at a process temperature of 670°C. FIG. 3 illustrates the light off curves of the calcined cobalt hydrotalcite catalyst of the present invention versus a Co-ZSM-5 catalyst in either trilobe or extrudate form. Experiments were conducted using a feed stream containing 10 N 2 O, 2% water, 2% oxygen and the balance nitrogen. Reactor pressure was maintained at 10 psig and gas hourly space velocity was maintained between 16,000 and 18,000 hr -1 . The Co-ZSM-5 catalysts are presented in U.S. Patent 5,171,553, issued December 15, 1992, and assigned to Air Products and Chemicals, Inc., Allentown, Pennsylvania, U.S.A. While the subject hydrotalcite catalyst and Co-ZSM-5 catalyst provide high conversion of N 2 O to gaseous nitrogen and oxygen, the calcined hydrotalcite catalyst demonstrates a light off temperature at least 75°C lower in temperature than Co-ZSM-5. The lower light off temperatures for the catalysts of the present process compared to prior art catalysts represents a significant process advantage in that energy requirements are reduced and less wear and stress are placed upon process equipment. FIG. 4 demonstrates that surface area of the catalysts according to the present process is substantially affected by heat treatment temperature employed during preparation of such catalysts. The data demonstrate that the optimum heat treatment temperature for achieving maximum surface area varies depending upon catalyst composition. For example, maximum surface area for Ni-Al hydrotalcite is achieved using a heat treatment temperature of about 220°C while maximum surface area for Co-Al hydrotalcite is achieved using a heat treatment temperature of about 320°C. The following examples are provided to further illustrate various embodiments of this invention and to provide a comparison between the enumerated catalysts of this invention and prior art catalysts for destroying N 2 O residing in gaseous mixtures containing oxygen and/or water. The examples are provided to illustrate the nature of the claimed process and are not intended to limit the scope of the claimed invention. Unless otherwise stated, parts and percentages in the examples are given by volume. GENERAL EXPERIMENTAL INFORMATION Inductively coupled plasma emission spectroscopy was used to analyze for M 2+ and M 3+ and Na + . X-ray diffraction patterns of the samples were taken using a Philips X-ray generator (Model PW1130) using CuK α radiation (λ = 1.5418 Å except for Co-containing compound wherein CoK α radiation (λ = 1.7902 Å) was used. TGA of these compounds were recorded on a Perkin-Elmer TGA7 instrument at temperatures ranging from 50-900°C at a heating rate of 20°C/min., under nitrogen atmosphere. The BET surface area of the catalyst powders was measured using a Carlo-Erba (Model 1800) sorptometer at 77°K. EXAMPLE 1 PREPARATION OF COBALT-ALUMINUM HYDROTALCITE HAVING A Co/Al MOLAR RATIO OF 2.0 A solution of 58.21g (0.20 mol) cobalt nitrate hexahydrate and 37.5g (0.10 mol) of aluminum nitrate nonahydrate in 70 cc of distilled water was added dropwise (over a 4-hour period at room temperature) to a solution containing 28.87g (0.70 mol) 97% NaOH, 20.03g (0.189 mol) sodium carbonate and 2.2 mol distilled water. The precipitate was stirred for 30 minutes, heated to 65°C for 16 hours, filtered, washed with large amounts of distilled water to remove excess sodium and nitrate, and dried at 110°C. 54.7% Co by weight, 11.2% Al by weight, Co/Al (atom) = 2.23, d(003) =7.58Å. XRD and IR confirmed that the compound prepared is single phase in nature with hydrotalcite-like structure. XRD showed sharp and symmetric reflections for (003), (006) and (110) planes indicating crystallinity characteristic of clay minerals possessing a layered structure. TGA studies showed two stages of weight losses. The first weight loss occurring between 150° and 200°C and the second occurring between 225° and 275°C. This result was substantiated by differential scanning calorimetry results which showed two endothermic peaks corresponding to the two weight losses. EXAMPLE 2 PREPARATION OF NICKEL-ALUMINUM HYDROTALCITE HAVING A Ni/Al MOLAR RATIO OF 2 A solution of 58.16g nickel nitrate hexahydrate and 34.51g of aluminum nitrate nonahydrate in 140 cc of distilled water was added dropwise (over a 4-hour period at room temperature) to 228 cc solution of 28.87g 97% NaOH and 20.03g sodium carbonate while maintaining the temperature at or below room temperature. The precipitate was stirred for 30 minutes, heated to 65°C for 16 hours, filtered, washed with large amounts of distilled water to remove excess sodium and nitrate, and dried at 110°C. 56.1% Ni by weight, 13.1% Al by weight, Ni/Al (atom) = 1.97; d(003) = 7.55 Å. EXAMPLE 3 PREPARATION OF COPPER-ALUMINUM HYDROTALCITE HAVING A Cu/Al MOLAR RATIO OF 2.0 A solution of 23.26g cupric nitrate pentahydrate and 18.76g of aluminum nitrate nonahydrate in 140 cc of distilled water was added dropwise (over a 3-hour period at room temperature) to a 114 cc solution of 14.44g 97% NaOH and 10.02g sodium carbonate while maintaining the temperature at or below room temperature. The precipitate was stirred for 60 minutes, heated to 65°C for 18 hours, filtered, washed with large amounts of distilled water to remove excess sodium and nitrate, and dried at 110°C. 66.0% Cu by weight, 12.6% Al by weight, Cu/A1=2.22. EXAMPLE 4 PREPARATION OF COBALT,MAGNESIUM - ALUMINUM HYDROTALCITE HAVING A Co/Mg/Al MOLAR RATIO OF 2/1/1 A solution containing Co 2+ , Mg 2+ and Al 3+ was made by dissolving 29.1g Co(NO 3 ) 2 6H 2 O (0.1 mol), 12.8g Mg(NO 3 ) 2 6H 2 O (0.05 mol) and 18.8g (Al(NO 3 ) 3 9H 2 O (0.05 mol) in 75 ml of deionized water at room temperature. A second solution was made by dissolving 14g NaOH (0.35 mol) and 10.6g Na 2 CO 3 (0.1 mol) in 100 ml of deionized water at room temperature. The second solution was slowly added (drop-by-drop) to the first solution at room temperature in a period of 1 hour with a vigorous stirring. The final pH of the precipitate suspension was 10. The precipitates were aged at 65°C for 18 hours with a good mixing. The resulting product was then filtered, washed with 300 ml of water in a beaker and filtered again. Finally, the material was dried in an oven at 110°C in air overnight. The yield was 18.0g. The anionic clay material prepared according to this method corresponds to the formula of the preferred embodiment wherein M is a mixture of cobalt and magnesium. Elemental analyses of this sample show 30.7% Co by weight, 6.05% Mg by weight and 7.12% Al by weight. The molar ratio of Co/Al=1.97 and Mg/Al=0.94. The d(003) spacings were found to be 7.69Å by X-ray diffraction. EXAMPLE 5 PREPARATION OF COBALT-RHODIUM, ALUMINUM HYDROTALCITE HAVING A Co/Rh/Al MOLAR RATIO OF 2/0.2/1 A solution of 1.0g (10% Rh) rhodium nitrate, 28.81g cobalt nitrate hexahydrate and 18.76g of aluminum nitrate nonahydrate in 114 cc of distilled water was added dropwise (over a half-hour period at room temperature) to a 114 cc solution of 14.44g 97% NaOH and 10.02g sodium carbonate while maintaining the temperature at or below room temperature. The precipitate was stirred for 2 hours, heated to 65°C for 18 hours, filtered, washed with large amounts of distilled water to remove excess sodium and nitrate, and dried at 110°C. The anionic clay material prepared according to this method corresponds to the formula of the preferred embodiment wherein M is cobalt and a portion of the aluminum has been replaced by rhodium. 58.3% Co by weight, 11.9% Al by weight, Co/Al=2.24, Rh/Al=0.016; d(003) = 7.54Å. EXAMPLE 6 CATALYTIC DECOMPOSITION OF N 2 O OVER CALCINED ANIONIC CLAY CATALYSTS The following general procedure was utilized for catalytically converting N 2 O to gaseous nitrogen and gaseous oxygen over the catalysts of the present invention. The reactor is was a Quartz, "U" tube having an inlet of 6,35 mm (1/4") O.D. and an outlet of 9,5 mm (3/8") O.D., wherein the catalyst was located at the outlet section. Typical catalyst weight used in these tests was 0.1 gram. The GHSV was 30,000h -1 . The reactor was surrounded by a temperature-controlled furnace and the temperature was monitored by a J-type thermocouple which was in contact with the catalyst bed. The concentration of N 2 O was 985 ppm balanced by He. Oxygen (2.5%) was added for some runs. H 2 O (2%) was added to the feed via a H 2 O saturator, and the level of H 2 O added can be controlled by adjusting the temperature of this saturator. The activity measurements were made using a microcatalytic reactor in a steady-state flow mode. Product analysis was obtained using an on-line gas chromatograph with a thermal conductivity detector. The separation column was packed Porapak Q (80/100 mesh) and was 1,219 m (4 feet) long having a 3,17 mm (1/8") outer diameter. Chromatograph oven temperature was 25°C and the flow rate of the carrier gas was 30 cm 3 /min. N 2 O decomposition experiments were carried out at ambient pressure and 0.1g of catalyst in a plug-flow reactor was used for each run. The flow rate of the feed was 100 cm 3 /min and remained constant resulting in a gas hourly space velocity (GHSV) of 30,000 h -1 . The reaction temperatures studied ranged from 200° to about 800°C. The catalysts studied were dried for one hour at 500°C over a helium stream prior to use. PRESENTATION OF DATA The catalysts according to Examples 1 through 5 and various prior art catalysts were tested according to the procedure of Example 6 for catalytic decomposition of N 2 O in the presence of oxygen. Reaction conditions were maintained constant using a feedstream of 0.1% N 2 O and 99.9% helium. A N 2 O-containing gaseous mixture containing 2.5% oxygen and 2% H 2 O was used as a feedstream in order to determine the effect of oxygen and water on catalyst activity. The results presented in Table 1 compare reaction conversion of N 2 O to gaseous nitrogen and oxygen obtained using calcined cobalt-aluminum hydrotalcite catalysts having varied ratios of cobalt and aluminum cation as a function of temperature. Runs 1 through 5, prepared according to the method of Example 1, present materials having a Co/Al ratio ranging from 1.1 to 3.55 which were used to decompose N 2 O residing in a helium stream containing 985 ppm N 2 O and alternately, 2.5% oxygen or 2.5% oxygen and 2% water vapor. The results demonstrate that N 2 O conversion for each of the runs increases with increasing reaction temperature with the best results obtained according to Run 4 which exhibits 84% N 2 O conversion at a reaction temperature of 450°C. Runs 3 through 5 demonstrate that the presence of oxygen or oxygen and water vapor in the N 2 O-containing gaseous mixture diminishes N 2 O conversion. Effect of Co/Al Ratio on N 2 O Decomposition Using a Cobalt-Aluminum Hydrotalcite Catalyst Calcined at 500°C Run Catalyst Co/Al Ratio BET Area (m 2 /g) N 2 O Conversion (%) 300°C 350°C 400°C 450°C 500°C 1 1.1 164 8 12 22 45 79 2 2.0 9 17 36 68 93 3 2.55 16 27 49 81(61 ,17 ) N/A 4 3.0 100 13 24 53 84(67 ,25 ) N/A 5 3.55 10 18 49 82(61 ,14 ) N/A Table 2 illustrates the effect of calcination temperature on catalyst activity for N 2 O decomposition using a cobalt aluminum hydrotalcite catalyst having a Co/Al ratio of 2.2. Runs 6 through 8, prepared according to the method of Example 1, were used to decompose N 2 O residing in a helium stream containing 985 ppm N 2 O and alternately, 2.5% oxygen or 2.5% oxygen and 2% water vapor. The results demonstrate that N 2 O conversion is maximized when the subject catalyst is calcined (heat treated) at a temperature ranging from 400° to 500°C. Runs 6 and 7 demonstrate that the presence of oxygen or oxygen and water vapor in the N 2 O-containing gaseous mixture diminishes N 2 O conversion. Effect of Calcination Temperature on Catalyst Activity for N 2 O Decomposition Using a Cobalt-Aluminum-Hydrotalcite Catalyst (Co/Al=2.2) Run BET Area (m 2 /g) Calcination Temp (°C) N 2 O Conversion (%) 300°C 350°C 400°C 450°C 500°C 6 50 400 45 79 94 99(97 ,43 ) N/A 7 500 47 74 93 99(96 ,54 ) N/A 8 10 800 5 5 9 12 27 The results presented in Table 3 compare reaction conversion of N 2 O to gaseous nitrogen and oxygen obtained using calcined anionic clay materials containing copper (Run 9), nickel (Run 10), rhodium (Runs 16 and 17) and cobalt, palladium (Run 12) versus conversion obtained using Co-ZSM-5 (Run 13), disclosed in U.S. Patent 5,171,553. The results demonstrate that the claimed process provides excellent conversion of N 2 O to gaseous nitrogen and oxygen. In particular, Run 17, a cobalt-rhodium aluminum hydrotalcite, prepared according to Example 5, provides 100% conversion of N 2 O to gaseous nitrogen and oxygen at a process temperature of 300°C, a full 150° lower than the 450°C temperature required for Co-ZSM-5 (Run 11) to yield comparable conversion. Moreover, Ni-Al-hydrotalcite (Run 10) provides substantially the same conversion to N 2 O at a process temperature of 450° when in the absence or presence of oxygen and water. When Al 3+ is partially replaced by La 3+ (Al/La = 1) (Run 19), the resulting material, Co-Al, La-HT, shows significantly high activity at low temperature (79% at 300°C). The enumerated catalysts of the present invention provide several improvements over prior art processes for removing N 2 O from combustion processes. First, the claimed catalysts are unexpectedly more active at lower process operating temperatures than prior art catalysts in converting N 2 O to nitrogen gas and oxygen gas. Second, the catalysts are not significantly deactivated in the presence of oxygen or water in the process feedstream.

Claims: A process for removing N 2 O from an N 2 O-containing gaseous mixture which comprises reacting the N 2 O-containing gaseous mixture in the presence of a catalyst under conditions sufficient to convert the N 2 O to gaseous nitrogen and gaseous oxygen, wherein the catalyst comprises an anionic clay material represented by the formula M m N n (OH) (2m + 2n) A a ·bH 2 O wherein M is a divalent metal cation; N is a trivalent metal cation; A is a mono-, di- or trivalent anion which decomposes when heated to a temperature sufficient to form a volatile gas; m and n are such that m/n has values of 0.5 to about 6; a is a number with the provisos that when A is a divalent anion, a = 1/2n and when A is a trivalent anion a = 1/3n and b is an integer having values of 1 to 10 which anionic clay material is first heated to the temperature sufficient to cause A, the mono-, di- or trivalent anion, to decompose to form the volatile gas and thereby convert the anionic clay material to the catalyst. The process according to Claim 1 wherein the anionic clay material is heated to a temperature ranging from 200°C to 800°C to form the catalyst. The process according to Claim 1 wherein the reacting is conducted at a temperature ranging from 100° to 800°C and pressures ranging from 0.1 to 300 atmospheres. The process according to Claim 1 wherein the reacting is conducted at a temperature ranging from 100° to 800°C and a gas hourly space velocity ranging from 1,000 to 300,000 hr -1 . The process according to Claim 1 where M is selected from the group consisting of Co 2+ , Ni 2+ , Cu 2+ , Pd 2+ , Zn 2+ , Mn 2+ , Mg 2+ and Fe 2+ and N is selected from the group consisting of Al 3+ , Ga 3+ , In 3+ , Fe 3+ , Rh 3+ , Co 3+ , B 3+ , Cr 3+ , La 3+ , Sc 3+ and Y 3+ . The process according to Claim 1 wherein N is selected from the group consisting of Ce 3+ , Nd 3+ , Pm 3+ , Sm 3+ , Eu 3+ , Gd 3+ , Th 3+ , Dy 3+ , Ho 3+ , Er 3+ , Tm 3+ , Yb 3+ and Lu 3+ . The process according to Claim 1 wherein the ratio of m to n recited in the formula ranges from 1 to 4. A process for removing N 2 O from an N 2 O-containing gaseous mature which comprises reacting the N 2 O-containing gaseous mixture in the presence of a catalyst under conditions sufficient to convert the N 2 O to gaseous nitrogen and gaseous oxygen wherein the catalyst comprises an anionic clay material represented by the formula M m Al 3+ n (OH) (2m + 2n) A a ·bH 2 O wherein M is Co 2+ , Cu 2+ , Ni 2+ , Pd 2+ or Mg 2+ ; A is a mono-, di- or tri-valent anion which decomposes when heated to a temperature sufficient to form a volatile gas; m and n are such that m/n has values of 0.5 to about 6; a is a number with the provisos that when A is a divalent anion, a = 1/2n and when A is a trivalent anion a = 1/3n and b is an integer having values of 1 to 10; which anionic clay material is first heated to the temperature sufficient to cause A, the mono-, di- or trivalent anion, to decompose to form the volatile gas and thereby convert the anionic clay material to the catalyst. The process according to Claim 8 wherein the anionic clay material is heated to a temperature ranging from 200°C to 800°C to form the catalyst. The process according to Claim 8 wherein the reacting is conducted at a temperature ranging from 100° to 800°C and pressures ranging from 0.1 to 300 atmospheres. The process according to Claim 8 wherein the reacting is conducted at a temperature ranging from 100° to 800°C and a gas hourly space velocity ranging from 1,000 to 300,000 hr -1 . The process according to Claim 8 wherein M as represented in the formula is copper. The process according to Claim 8 wherein M as represented in the formula is cobalt. The process according to Claim 8 wherein M as represented in the formula is nickel. The process according to Claim 8 wherein M as represented in the formula is palladium and cobalt. The process according to Claim 8 wherein M as represented in the formula is cobalt and magnesium. The process according to Claim 8 wherein M as represented in the formula is cobalt and zinc. The process according to Claim 8 wherein the anionic clay material is cobalt-lanthanum aluminum hydrotalcite. The process according to Claim 8 wherein the anionic clay material is cobalt-rhodium aluminum hydrotalcite.

Inventors: ARMOR JOHN NELSON, BRAYMER THOMAS ALBERT, KANNAN SRINIVASAN, LI YUEJIN, SWAMY CUNCHALA SUBRAMANYA, ARMOR, JOHN NELSON, BRAYMER, THOMAS ALBERT, KANNAN, SRINIVASAN, LI, YUEJIN, SWAMY, CUNCHALA SUBRAMANYA
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1994
Main classification: B01D53
IPC codes: B01D53, B01J21, B01J20, B01J23, C01B21